Conditions of abnormally increased intracranial pressure are often treated surgically by the implantation of an artificial tube, or shunt, to conduct excess fluid from the intracranial space to another part of the body, for example, to the peritoneum or right atrium of the heart. These shunt tubes are typically non-collapsible. A differential pressure valve is often inserted in series with the tube, to maintain upstream pressure at least a certain amount above downstream pressure.
However, in the upright posture, the downstream pressure can drop low enough that the upstream, intracranial pressure, drops to abnormally low values, despite the differential pressure valve.
Attempts to solve this problem have fallen into two classes. Differential diaphragm valves allegedly reference atmospheric pressure to prevent siphoning. These, however, have a complicated structure, subject to clogging with tissue debris, and their exterior design, of limited area, is subject to interference by scar tissue. Gravity sensitive valves, the second class, attempt to sense the angular position of the body with a weight or other device, but they are only sensitive to body position at one location, and are often non-linear.